The present invention relates to a method for providing coordination among multiple call switching centers in a wireless communication service.
Wireless communication service is well known. Typically, call traffic is exchanged between a mobile station and a base station via a pair of radio channels. The mobile station and base station communicate via one of many defined formats, such as FM, TDMA or CDMA. The base station connects to the well-known public switched telephone network ("PSTN") and, in some embodiments, routes calls to a call switching center ("CSC" or "call handler"). End to end communication between two parties may extend from a caller at the mobile station through the base through and PSTN to the CSC; the call extends back from the CSC through the PSTN to a called party.
FIG. 1 illustrates a CSC-based wireless system used in an air-to-ground communication network. There, the mobile station 110 (an "air terminal") typically is provided on an aircraft. It communicates with one of a plurality of ground stations 120-170. As the air terminal 110 travels, it may hand-off an active call from one ground station to another. According to this procedure, when the air terminal 110 determines to hand-off, it seizes a radio channel of a new ground station 130 and releases the radio channel of an old ground station 140.
The air-to-ground communication network 100 includes only one CSC 210, provided in the PSTN 200. Thus, the PSTN 200 routes calls from the ground stations to the CSC 210 and further to the called party. The CSC 210 manages calls received from mobile stations in the wireless air-to-ground communication network for authentication and billing purposes.
In such a system, a new call begins when a mobile station 110 seizes an available radio channel of a nearby ground station 140. The ground station 140 establishes a call path through the PSTN 220 to the CSC 210. If a hand-off occurs during the call, the mobile station 110 seizes a radio channel of a new ground station 130. The mobile station 110 and the new ground station 130 exchange call setup signaling by which the ground station 130 may determine that the call has arrived to the ground station 130 through hand-off rather than arriving as a newly originated call. The new ground station 130 routes the call to the CSC 210 and signals the CSC 210 to indicate that the call arrived by hand-off. Because a single CSC 210 is provided for the network, the ground station 130 may be programmed to route all calls received by hand-off to the CSC 210. In response, the CSC 210 drops the call path extending from the old ground station 140 in favor of the call path from the new ground station 130.
Wireless communication services are extremely successful. With continued success, consumer capacity will exceed the capacity of a single CSC. Multiple CSC's will be needed. The current hand procedure for handling calls received by a ground station hand-off cannot be extended to a multiple CSC system. The ground station will not know when CSC handling the calls prior to its arrival at the ground station. Accordingly, there exists a need in the art for a coordination scheme in a wireless network that maintains the integrity of communication links between a mobile subscriber and a called party during hand-off.